1. Field of the Invention
The present invention is directed to a defibrillator of the type having a charging capacitor that is connectable to a charging circuit by switches at both sides of the capacitor for charging and, for defibrillation of a heart, is connectable via a controllable switch means to at least two electrodes arranged in the region of the heart.
2. Description of the Prior Art and Related Subject Matter
A defibrillator of this type is disclosed in U.S. Pat. No. 4,800,883 for implantation in the body of a patient. This known defibrillator contains a charging capacitance composed of two capacitors that are connected to a charging circuit so as to be charged to a prescribed voltage. The two capacitors are also connected to two electrodes placed at the heart of the patient via a switch arrangement composed of four switches arranged in a bridge circuit. For defibrillation of the heart, the charging capacitance is first charged to a prescribed voltage and is subsequently connected via the switch means to the electrodes at the heart, so that the charging capacitance discharges across the heart tissue with a discharge current. By controlling the four switches of the switch means to open and close in pairs, the discharge current through the heart tissue is divided into a plurality of sub-currents that follow one another with alternating direction of the current.
The current through the heart tissue that produces the defibrillation is dependent on the charging voltage of the charging capacitance and on the electrical resistance of the heart tissue between the electrodes. The current has its highest value at the beginning of the discharge of the charging capacitance and then exponentially decays. In order to obtain an effective defibrillation of the heart, the current through the heart tissue must exceed a specific minimum value over a defined duration. For this reason, the charging voltage for the charging capacitance is selected such that the current is adequately high at the beginning of the charging event so that it drops below the minimum value only after the end of the defined duration. That part of the current exceeding the minimum value does not result in a more effective defibrillation (since effective defibrillation already occurs as soon as the minimum value is reached) and may lead to damage to the heart tissue due to the high initial value.
European Patent Application No. 92107996.8 discloses a defibrillator of the type initially cited, wherein a means for limiting the current for a prescribed maximum value is provided.
German OS 30 20 479 discloses a relay wherein by a defined quantity of a conductive fluid such as, for example, mercury is arranged between two contacts and is under the influence of an electro-mechanical transducer in the form of a piezoelectric drive element that can be deflected by electrical drive from a quiescent position into a working position. In the working position of the piezoelectric drive element, the liquid is deformed opposite the effect of its surface tension such that it produces a connection between the contacts.